Lithium greases have been known and widely used due to their advantages which include high water resistance and ease of dispersion, in all types of lubricating oil base stocks. Generally, lithium soaps used as thickening agents for lithium complex greases have been prepared by reacting lithium hydroxide or other lithium bases with conventional high molecular weight fatty acids. However, lithium 12-hydroxystearic acid and related lithium salts have been particularly preferred because of their great mechanical stability.
The fields of application for lithium complex greases having a high dropping point are extensive. By means of example, lithium complex greases have been successfully used for the lubrication of traction motor bearings, automotive disc brake wheel bearings and the like, particularly for automobiles and locomotives. In many cases the bearings are required to operate for a long period of time without maintenance at temperatures as high as 250.degree. F. and thus require particularly high dropping point greases for smooth operation.
Lithium soap greases are known in the art which utilize lithium salts of hydroxyfatty acids and other components.
U.S. Pat. No. 3,929,621 to Murray et al. discloses a grease composition including a dilithium salt of a C.sub.4 -C.sub.12 dicarboxylic acid, a lithium soap of a 9-, 10- or 12-hydroxy (C.sub.12 -C.sub.24) fatty acid such as lithium 12-hydroxystearate, and a lithium salt formed in situ in the grease from a second hydroxycarboxylic acid.
U.S. Pat. No. 4,435,299 to Carley et al. discloses a grease composition including the lithium salt of a C.sub.12 -C.sub.24 hydroxyfatty acid such as 12-hydroxystearic acid in a dilithium soap of a C.sub.4 -C.sub.12 dicarboxylic acid.
However, none of the prior art patents utilizes aromatic dicarboxylic acids. By means of example, Murray et al. (column 2, lines 13-15) and Carley et al. (column 2, lines 66-68) disclose that suitable acids include succinic, glutaric, adipic, suberic, pimelic, azelaic, dodecanedioic and sebacic acids. These are all linear acids.
Among linear acids, the prior art has preferably utilized sebacic and azelaic acids, and more preferably azelaic acid. The processing of dioctyl sebacate precursor is difficult and produces noxious vapors, e.g., 2-ethylhexanol. Moreover, azelaic acid is not always readily available in the marketplace and therefore other products capable of substituting for it are needed.
U.S. Pat. No. 2,874,121 to Hotten describes grease compositions which are thickened by methyl soaps of esters of terephthalic acids. This prior art patent describes a series of metal salts which are useful including all monovalent metals of Groups I, II, III, and IV of the Mendeleeff Periodic Table. Lithium is mentioned as one of various metals but the preferred claimed metals are sodium and barium. Moreover, the terephthalic derivatives utilized by Hotten are either mixed monoalkylester-monometal salt terephthalate derivatives or mixed monoalkylester-monoalkylamide terephthalate derivatives. No mention is made by Hotten of dilithium salts from dialkyl terephthalates or to a potential use thereof as a thickener for lubricating oils. Nor is it mentioned in the Hotten patent that terephthalate acid derivatives are to be added to a lithium 12-hydroxystearate compound for thickening greases.
Accordingly, there is a specific need for an improved grease thickener which when added to at lubricating oil base stock not only affords good mechanical stability, water resistance and lubricating capacity, but in addition imparts to a lubricating oil stock to which it is added a high dropping point which enables its performance at particularly high temperatures for prolonged periods of time.